In certain instances it is necessary to switch or otherwise control the application of large amounts of electrical power that is delivered to a device, such as an electric furnace. Two types of switching equipment may be utilized to achieve this end—a circuit breaker or a load break switch. A circuit breaker is a switching device, which is designed to interrupt over currents on an infrequent basis. In fact, the circuit breaker is not designed to provide routine switching of power, and has poor reliability and reduced operating life if used in such a manner.
The load break switch, such as a vacuum interrupter switch, on the other hand is designed for frequent interruption, or switching of electrical currents, and is configured to be highly reliable over its operating life. However, load break switches generally have lower current ratings as compared to circuit breakers. As such, load break switches are normally configured in parallel to achieve a higher current rating. To achieve this increased current rating, a complex control system is required to coordinate the switching of the parallel load break switches.
Three-phase power is a type of power in which the total power is distributed evenly in each individual phase. Thus, the switching of three-phase power requires that each individual phase be switched separately by at least one switching element for each phase. However, when load currents exceed the current rating of the switch, multiple switches arranged in parallel may be utilized in each phase to increase the overall current rating of the switches. For example, a 30 MVA arc furnace with a 34 KV primary voltage requires 500 amperes of current for each of the three phases. Thus, to switch the 500 ampere current in each phase, three 500 ampere rated load switches (one switch per phase) would be needed to switch the current supplied to the furnace. In another example, a 60 MVA arc furnace with a 34 KV primary requires 1000 amperes of current for each of the three phases. As such, to accommodate the increased load current, two of the 500 ampere load switches would need to be placed in parallel in order to accommodate the 1000 ampere load current supplied to the furnace. In other words, two load switches per individual phase of the three-phase load, for a total of six load switches, are required to switch the supplied load current. In fact, a 90, 120, 150, or a 180 MVA furnace would require a total of nine, twelve, fifteen, or eighteen load switches respectively to switch the required load currents corresponding to 1500, 2000, 2500, or 3000 amperes.
Because load switches are generally used to switch three-phase power, the switches are arranged in multiples of three (one switch or groups of parallel switches per phase) to achieve the desired current rating for each phase. However, prior art switchgear controls used to coordinate the operation of multiple load switches are typically uniquely designed for each furnace, or individual application. As such, little commonality was provided between each of the switchgear controls. The unique design of the switchgear control, and the lack of commonality makes prior art switchgear controls highly complex, and difficult for technicians to analyze and troubleshoot. For example, when the switchgear control fails, the technician is often required to expend a substantial amount of time in gaining an understanding of the circuit layout of the control before an analysis can be performed.
Additionally, when multiple load switches are operated, as previously discussed, very large currents are required to control each of the load switches. For example, a single solenoid actuated load switch may require during its switching operation, 70 amperes of current at 120 volts. When multiple solenoid actuated load switches are utilized, the required control current increases greatly. However, in some remote areas, the large currents needed to operate the solenoid actuated load switches are not available, thus making prior art switches useless in such circumstances.
The manner in which electrical currents are switched may also have a impact on the longevity and the overall performance of the electrical device to which power is supplied. For example, when a transformer for a furnace is initially turned on, an electrical current passes through the windings, which may be many times greater in magnitude than the normal load currents. These large currents generate magnetic forces on the windings, which wears or degrades the winding's insulation. After the insulation on the windings fails, the transformer must be rewound, which is both costly in terms of lost production and repair costs.
Finally, when numerous parallel load switches are utilized for an application, it may become difficult to identify, and replace a particular load switch that has failed, or has nearly reached a failed state. Typically, the failed load switch is identified by removing all of the bus conductors to isolate and test each load switch individually. Unfortunately, such a process requires the expenditure of a tremendous amount of time and labor.
Therefore, there is a need for a modular switchgear control that can be configured in a modular manner with one or more line powered control panels, and low energy control panels that can be easily removed from a cabinet and replaced in a cabinet. Additionally, there is a need for a modular switchgear control that is scaleable, and can be configured with multiple line powered or low energy control panels so that one or more vacuum interrupter switches having reduced power ratings can be combined to achieve a higher overall current rating. Still yet there is a need for a need for a method of operating a low energy panel using capacitors. In addition, there is a need for an automated noise detection system that identifies when a vacuum switch has failed.